1. Field of the Invention
The invention is directed to imaging systems, and more particularly to radiographic X-ray imaging systems, for medical, industrial, and other applications.
2. Description of Related Art
Radiographic X-ray imaging systems for medical, industrial and other applications typically use a point-source X-ray tube in which energetic electrons impinge upon a solid metal target thereby producing a cone-beam of X-ray light emanating from the focal spot. The spectrum of X-rays emitted from such tubes is poly-energetic, having line emission characteristic of the anode material used in the tube (commonly tungsten, or in the case of mammography, molybdenum or rhodium) superimposed on a broad continuum of Bremsstrahlung radiation extending to a high-energy cutoff determined by the applied voltage. For many imaging tasks, however, increased image contrast—and lower patient dose, in the case of medical applications—can be achieved using mono-energetic radiation.
One method for producing (nearly) mono-energetic radiation from electron-impact X-ray tubes (or other point-sources of X-rays) utilizes multilayer X-ray mirrors to reflect and filter the X-ray light before it reaches the tissue or sample under study. [See, for example, ‘X-ray monochromator for divergent beam radiography using conventional and laser produced X-ray sources’, H. W. Schnopper, S. Romaine, and A. Krol, Proc. SPIE, 4502, 24, (2001)]. The X-ray mirrors include flat substrates coated with X-ray-reflective multilayer coatings that reflect X-rays only over a narrow energy band. The multilayer X-ray mirrors are positioned between the X-ray tube focal spot and the sample or patient. Because the mirrors only work at shallow grazing incidence angles, a single mirror will only yield a thin fan-beam of mono-energetic X-ray light. Thus, to produce mono-energetic light over a large field at the image plane, one of two approaches can be used. In the first approach, a single mirror is scanned over a wide angular range during the X-ray exposure. In the second approach, an array of stacked mirrors are used, constructed from a number of thin mirrors and spacers that are stacked together with high precision in a wedge shape: while each individual mirror will produce a narrow fan beam, the array of mirrors will collectively produce an array of co-aligned fan beams. In the second approach using a mirror stack, however, the illumination pattern will also include dark strips corresponding to the regions where the X-ray light is blocked by the edges of the mirrors. To compensate for the dark strips, the mirror stack can be scanned during exposure, similar to the way in which a single mirror is scanned in the first approach (albeit over a much smaller angular range), so that the bright and dark strips are averaged together to produce uniform illumination.
In any case, the requirements on positioning the mirrors relative to the focal spot are stringent: in particular, the angular position of each mirror must be such that the incidence angle of X-rays is controlled to a fraction of a degree. As an example, in the specific case of multilayer X-ray mirrors designed for mammography systems operating near 20 keV, approximately, typical grazing incidence angles are in the range of 0.3-0.7 degrees, while the angular acceptance angle of the narrow-band multilayer coating can be as small as 0.02 degrees; therefore the mirror must be positioned so that the error in graze angle is perhaps half of the acceptance angle, i.e., 0.01 degrees, or less. For other types of X-ray imaging systems utilizing higher-energy X-rays, the graze angles and acceptance angles are even smaller, and thus the requirements on alignment are even more stringent than for mammography.
For either approach using X-ray mirrors just described, i.e., using a single mirror or a mirror stack, a precision scanning mechanism is required for illumination over a large field. Such a scanning mechanism must be constructed such that the alignment of the mirror or mirror stack relative to the X-ray focal spot is precisely maintained during the course of the scan, the scan range must be accurately controlled (i.e., to a small fraction of a degree), and the scanning mechanism must be highly repeatable so that no exposure errors are introduced. The scanning mechanism must be constructed so that the rotation axis can be made to coincide with the focal spot with a precision that is determined by the size of the focal spot and by the angular acceptance of the multilayer mirrors. For mammography, for example, the displacement error between the rotation axis and the focal spot must be smaller than 0.05 mm, approximately. For other imaging applications, this displacement error may be larger or smaller.
In summary, while the notion of using multilayer X-ray mirrors in conjunction with point-source X-ray sources to produce mono-energetic X-rays for radiographic imaging has been described previously, no mechanism has yet been developed to accurately and precisely mount, align, and scan the mirrors.